1. Field of the Invention
This invention relates to multiplex serial loop transmission systems and, more particularly, to fault isolation and partial service restoration in such systems.
2. Description of the Prior Art
J. P. Forde et al U.S. Pat. No. 3,458,661 granted July 29, 1969, and L. Hochgraf U.S. Pat. No. 3,519,935, granted July 7, 1970, each disclose a serial loop transmission system in which partial service restoration is effected by looping the repeatered line back to a central terminal on the central terminal side of detected faults. This permits maximum possible service with the faulty system and isolates the location of the fault to speed up repair and restoration of full service.
One difficulty with the systems described in the aforementioned patents is the wide variances in the start-up time for the various different configurations and different loop lengths of the looped-back system. If sufficient time is not allowed for full system start-up before testing the system for faults, a fault can be indicated when one has not, in fact, occurred. Moreover, if a remote terminal initiates a loopback in response to a fault on the outgoing link before the central terminal is able to detect that fault, the central terminal is unable to pinpoint the location of the fault and provide an indication to service personnel of that location. Such an unrecognized fault can lead to an oscillatory condition where the central terminal continually tries to add a link while the remote terminal continually attempts to remove that link.
Yet another problem in the prior art loopback systems arises when portions of the system are powered from remotely located power supplies. Attempts to locate faults beyond the remote powering position cause a failure due to the lack of a complete powering path which, in turn, masks the locations of actual faults in the transmission system beyond that point.